Squat, dark, and powerful, Alvin Siglos had been the leader of a rambunctious gang at Basilan High School three decades earlier. The boys hung together constantly, playing sports during the day and gathering to raise hell at night. Siglos was the soccer team’s captain and best player, and Tilao was a pugnacious defenseman. Reckless and fun-loving, Tilao seemed as far from serious thought about politics or religion as a young man could be. He skipped classes and got in trouble for fighting. His father was an honorable, pious Muslim, but Tilao was not. He and the other boys liked girls, and they liked to drink, gamble, and fight.

Siglos had not strayed far from home or from his youthful pursuits. Sabban found him living in Zamboanga City, a sprawling metropolis just a short boat ride from Basilan at the southwestern end of the large island of Mindanao. He was a cheerful, garrulous man with uneven skin, thick black hair, and a wide face with narrow dark eyes set wide apart. He didn’t look like an athlete, but he was a star in the local softball leagues and was considered one of the city’s best tennis players. He had little education and no interest in religion or politics. He worked only when the need was strong and a suitable opportunity presented itself; he was the kind of man who lived off hustle and charm. He was also an enthusiastic gambler, wagering heavily on the hugely popular Zamboanga cockfights. He knew that his old high-school friend had moved to Saudi Arabia years earlier, and he’d learned only by accident that Tilao was back in the Philippines: One day in 2000 he heard someone sounding like Tilao boasting on the radio, though the voice had been identified as belonging to someone named “Abu Sabaya.” He was curious enough to ask one of Tilao’s cousins, and was told that his old soccer buddy was now the most notorious jihadist in the Philippines. Siglos began boasting that he was the outlaw’s “best friend.”

Months later, word apparently reached Tilao that his old friend had been talking up their relationship, and out of the blue the terrorist leader surprised Siglos with a phone call.

“Are there lots of millionaires there in Zamboanga City to kidnap?” he asked.

“No,” Siglos told him. “We are all poor.”

Another surprise call came after the Dos Palmas kidnapping, and this time Tilao wanted help. He asked Siglos if he had a bank account. He was looking for someone he could trust, someone outside known Abu Sayyaf circles, to collect and hold ransom money. Siglos said he had never had enough money to need a bank.

“I will call you again,” Tilao promised.

These furtive contacts with his friend on the run were thrilling for Siglos, and at first he didn’t waste much time thinking about the nature of the Abu Sayyaf’s crimes, or about its victims. Their childhood friendship transcended such abstract considerations, and he was inclined to help Tilao if he could. Then came the Golden Harvest kidnapping. What Tilao was doing no longer seemed so abstract.

It was at this moment that Siglos was first contacted by an operative of Colonel Sabban’s. The marine asked Siglos if he wanted to work as an “action agent.” Apart from the pay he would receive, the Philippine government was offering a $100,000 reward for information leading to Tilao’s capture or death. (The Americans would later put up a reward of $5 million. Tilao scoffed at the local reward, which he considered too small, but was proud of the American one.) At some level Siglos still loved his old friend; and he would later see that Tilao still trusted him. But as Siglos figured it, not even their mutual friends could fault him for informing on the outlaw. Nor could they accuse him of selling out his friend for money: Blood was thicker than friendship. Revenge weighed heavily on the scale. Siglos was so eager to cooperate that the marine intelligence unit was at first taken aback.

Video Clip:Captain Gieram Aragones describes his initial distrust of Siglos (Press the play button to begin.)

Captain Gieram Aragones was unimpressed after the initial interview. He didn’t trust Siglos. A wiry man with pale skin, Asian eyes, and the nickname “Bong,” Aragones was obsessed with getting Tilao. It was the most important job he had been given, and apart from his ambition and professional pride, he found the Abu Sayyaf personally offensive, and Tilao particularly so. The son of a marine chaplain and a graduate of the Philippine naval academy, Aragones was a convert to Islam. He had taken well to undercover intelligence work, immersing himself totally in the culture and languages of Sulu province. To him, Abu Sayyaf was an illegitimate political movement and a bogus religious one. Tilao and the other jihadists gave his adopted religion a bad name. As an outward symbol of his determination, Aragones announced to his men that he would neither cut his hair nor shave until Tilao was either in custody or dead.

Siglos was a potentially important avenue to Tilao, but Aragones judged that anyone who talked so much and displayed such transparent lust for reward money was dangerous. The would-be informant boasted that he had worked many times as an undercover agent for the Philippine organized-crime task force, but his claims didn’t check out. Aragones saw him as a blowhard and an opportunist. Before putting Siglos on the payroll as an action agent, Aragones recommended to Colonel Sabban that the recruit be put to a test.

Mark Bowden is a national correspondent for The Atlantic, and a best-selling author. His book Black Hawk Down, a finalist for the National Book Award, was the basis of the film of the same name. His book Killing Pablo won the Overseas Press Club's 2001 Cornelius Ryan Award as the book of the year. Among his other books are Guests of the Ayatollah, an account of the 1979 Iran hostage crisis, which was listed by Newsweek as one of "The 50 Books for Our Times." His most recent books are The Best Game Ever, the story of the 1958 NFL championship game, and Worm, which tells the story of the Conficker computer worm, based on the article "The Enemy Within," published in this magazine.

Mark has received The Abraham Lincoln Literary Award and the International Thriller Writers' True Thriller Award for lifetime achievement, and served as a judge for the National Book Awards in 2005. He is a 1973 graduate of Loyola University Maryland, where he also taught from 2001-2010. A reporter and columnist for The Philadelphia Inquirer for more than 30 years, Bowden is now an adjunct professor at The University of Delaware and lives in Oxford, Pennsylvania. He is married with five children and two granddaughters.